Order- disorder phase transition temperature from L2(1) to B2 phase, T-t(L21/B2), in Co-based Heusler alloys was systematically investigated based on the Braggs-Williams-Gorsky (BWG) approximation. Because height of T-t(L21/B2) closely correlates to phase stability of the L2(1) phase, conformation of T-t(L21/B2) would be a key to obtain a highly ordered L2(1) phase. From modification with the BWG approximation it was shown that the degree of order indicated temperature dependence, namely, the degree of order decreased above around 0.6 T-t(L21/B2) and reached zero at T-t(L21/B2) . Therefore, it should be better to select the Co-based Heusler alloys with a higher value of T-t(L21/B2) t if the higher degree of order in the L2(1) phase is desired by thermal annealing at as low temperature as possible. From powder neutron di r raction (ND) studies of Co2Y Ga (Y = Ti, V, Cr, Mn and Fe) alloys it was actually conformed that materials with higher value of T L2(1)= B2 t indicated higher degree of order even in the same annealing temperature. Studies for the phase state in Co-Ti- Fe-Ga films also showed that a height of T-t(L21/B2) concretely correlated to the phase stability of the L2(1) phases, that is, the specimen with higher value of T-t(L21/B2) t was easy to obtain with a higher degree of order.